Crack it open and let it air dry for a day, maybe there is still water inside.
Examing all areas for traces of flux residue that the water may have caused to pool in certain areas like around the power switch. It may have dissolved some metal from the components and made a conductive path, which you can get rid of by flushing with a mild detergent solution and a tiny paintbrush on the area. Remember that with a soldered on battery you risk shorting it, a low current path while wet so do not leave it wet long, ideally if the battery could be unplugged you would unplug it first but it can't be - up to you whether to consider unsoldering it.
After a detergent solution or even plain distilled water, follow up with a final rise with alcohol. If you have no 100% pur alcohol, something like rubbing alcohol that is 70%/30% alcohol/water will be fine, just shake out excess liquid when finished and be sure it's 100% dry before you attempt power-on again. The alcohol containing rinse is to decrease the surface tension of the water, like detergent does, but it also evaporates away completely which the detergent does not, significantly reducing the drying time and the changes that water soluble flux remaining in trace amounts on electronics, will collect in high concentration where beads of plain water remain while it dries. Think of it like water spots on a car, the contaminants in the water become more and more concentrated as the water drop shrinks while it dries because it has surface tension high enough to bead..